Several
types of Garnets with optical phenomena are known. They
include color-change, cat's eye, star and rarely, iridescent
Garnet. Color change Garnets are usually Pyrope-Spessartine,
while star Garnets are generally Almandine or
Pyrope-Almandine.
Iridescent
or "Rainbow" Garnet is a very unique variety
of Andradite or
Grossular-Andradite
(also called “Grandite”)
Garnet. Specimens of iridescent Garnet were first collected
in 1934 at the Adelaide mining district, Nevada, USA.
Early studies of these samples suggested the iridescence
was due to periodic twinning under the crystal surfaces
but later, more in-depth investigation did not show
any evidence of twinning.

Rainbow
Garnet has also been found in Mexico at a mine located about 200 kilometers southeast of Hermosillo in Sonora, Mexico.
This was the main source of iridescent Garnet until
the discovery in Japan in 2004. Rainbow Garnet from
Japan was first found in early 2004 by several
groups of mineral collectors near the old Kouse Magnetitemine in the
Tenkawa area of Nara Prefecture located approximately
60 km southeast of Osaka. The Garnets found here
were not high quality but in September 2004 a local
mineral collector named J. Sugimori found an outcrop
containing gem-quality, strongly iridescent and relatively
clean Andradite Garnet crystals. Over the next four
months, he used hand tools to follow the vein to a depth
of 5 m. Several hundred kilograms of well-formed Rainbow
Garnet crystals showing spectacular iridescence were
recovered. A small portion of this material could
be considered gem quality.
In August 2005, local authorities
from Tenkawa village prohibited additional Garnet recovery
from the area. This source
is now closed and off-limits and
mostly overgrown by forest. Collecting at
this location is strictly prohibited and
field collectors have been arrested.

The
Rainbow Garnet from this location occurs as relatively small
crystals with an orangy brown body color but exhibit
a variety of iridescent colors depending on the viewing
orientation and the particular crystal face. The phenomenon
varies from a single color to a range of colors that
shifts as the gem is rotated. Some crystal faces
can even exhibit a full spectrum of colors, or a "rainbow"
of colors. The crystals from this location are nearly pure Andradite, as
determined by their chemical composition. Microscopic examination shows the presence
of a rather complex lamellar, or thin-layered, structure.
These alternating, microscopic layers are of slightly
different composition and varying widths. Some layers
are nearly pure Andradite, others are Andradite with
a Grossular component. These microscopic layers
were formed by crystal intergrowth, distortion and possibly
twinning. This complex structure is responsible for
the iridescent colors by diffraction of light through
and bouncing off of the many microscopic layers. (GIA Gems & Gemology,
Winter 2006)

Rainbow Garnet crystals are beautiful
and popular but faceted gems are very rare. This is due to the thin
layers of iridescence being so close to the
surface of a crystal. Polishing of facets too deeply
will polish the iridescence right off. Also,
only a very small percentage of
the crystals are facetable. Most crystals are completely opaque. A translucent gem
would necessarily be small. A gem
with nice symmetry and translucency is quite rare. The gem pictured above is
a rare and beautiful example of a small, translucent
gem with excellent rainbow iridescence. As the gem is rotated the various facets
light up with the iridescent rainbow flash.

Rainbow
Garnet
gems for sale:

We
have not photographed our Rainbow Garnet
gems
yet. Please
check back soon.